Update ByteBuffAllocator.java

[moonlightingLL]

In ByteBuffAllocator.java, there is no value checking for poolBufSize and this variable is directly used to calculate the bufsForTwoMB. When poolBufSize is mistakenly set to 0, the code would cause division by 0 and throw ArithmeticException to crash the system.

public static ByteBuffAllocator create(Configuration conf, boolean reservoirEnabled) {
    int poolBufSize = conf.getInt(BUFFER_SIZE_KEY, DEFAULT_BUFFER_SIZE);
    if (reservoirEnabled) {
    . . .
    int bufsForTwoMB = (2 * 1024 * 1024) / poolBufSize;
    . . .

I add a piece of code to check whether poolBufSize is equal to 0 during runtime, and if it's 0, it will raise an IllegalArgumentException exception with the given message.